Requirements for ever-thinner thin film deposition, improved uniformity over larger surfaces, and higher production yields have been, and still are, the driving forces behind emerging technologies developed by the research community and commercialized by equipment manufactures for coating wafers to make electronic devices. As these devices become smaller and faster, the need for improved uniformity and better defined layer thickness, as well as film properties such as conductivity and the like, rises dramatically.
Various technologies well known in the art exist for applying thin films to wafers or other substrates in manufacturing steps for integrated circuits (ICs). Among the more established technologies available for applying thin films, Chemical Vapor Deposition (CVD) is an often-used commercialized processes. Atomic Layer Deposition (ALD), a variant of CVD, is flow emerging as a potentially superior method for achieving uniformity, excellent step coverage, and cost effective scalability to substrate size increase. ALD however, can exhibit a generally lower deposition throughput (typically <100 {hacek over (A)}/min) than CVD, but is suitable for ultrathin films, less than typically 100 {hacek over (A)}.
CVD is a flux-dependent technique requiring specific and uniform substrate temperature and stringent uniformity of precursors (chemical species) flux in order to produce a desired layer with uniform thickness and properties on a substrate surface. These stringent requirements become more challenging as substrate size increases, sometimes dictating additional chamber design complexity and manifold complications to sustain adequate film uniformity and properties. Another problem in CVD coating, wherein reactants and the products of reaction coexist in a close proximity to the deposition surface, is the probability of inclusion of reaction products and other contaminants in each deposited layer. Still further, highly reactive precursor molecules contribute to homogeneous gas phase reactions that can produce unwanted particles, which are detrimental to film quality and device performance.
Another critical area of thin film technology is the ability of a system to provide a high degree of uniformity and thickness control over a complex topology, referred to as step coverage. In the case of CVD, step-coverage typically exceeds typical physical vapor deposition (PVD) performance. However, certain disadvantages of CVD make ultrathin CVD films inadequate for many emerging critical semiconductor applications. For example, film initiation via nucleation deems CVD films discontinuous and practically useless for many sub 50 {hacek over (A)} needs. Likewise, coating high aspect ratio features with conformal CVD films while maintaining film quality and adequate throughput is difficult.
ALD, although at present a slower process than CVD, demonstrates a remarkable ability to deposit uniform, ultra-thin films over complex topology. This robust and inherent property comes from the flux independence of ALD. In addition, ALD implementation requires time and space separated molecular precursors which in turn circumvents gas phase reactions and therefore enables utilization of highly reactive precursor. Accordingly, ALD process temperatures are typically and advantageously lower than typical conventional CVD process temperatures.
ALD processes are executed by a series of self saturating surface processes. Generally, ALD is a process wherein conventional CVD processes are divided into single-monolayer depositions, in which each separate deposition step theoretically goes to saturation at a single molecular or atomic monolayer thickness and self-terminates when the layer formation occurs on the surface of a material. Generally, in the standard CVD process, the precursors are fed simultaneously into a reactor. In an ALD process the precursors are introduced into the reactor separately at different steps. Typically the precursors are introduced separately by alternating the flow of the precursor to combine with a carrier gas being introduced into the reactor while coexistence of the precursors in the reactor is suppressed by appropriately purging or evacuating the reactor in between successive introduction of precursors.
For example, when ALD is used to deposit a thin film layer on a material layer, such as a semiconductor substrate, saturating at a single molecular or atomic layer of thickness results in a formation of a pure desired film and eliminates the extra atoms that comprise the molecular precursors (or ligands). By the use of alternating precursors, ALD allows for single layer growth per cycle so that much tighter thickness controls can be exercised to deposit an ultra thin film. Additionally, ALD films may be grown with continuity with thickness that is as thin as a monolayer (3-5 Angstroms). This capability is a unique characteristic of ALD films that makes them superior candidates for applications that require ultrathin films. A good reference work in the field of Atomic Layer Epitaxy, which provides a discussion of the underlying concepts incorporated in ALD, is Chapter 14, written by Tuomo Suntola, of the Handbook in Crystal Growth, Vol. 3, edited by D. T. J. Hurle, © 1994 by Elsevier Science B. V. The Chapter title is “Atomic Layer Epitaxy”. This reference is incorporated herein by reference as background information.
The unique mechanism of film formation provided by ALD offers several advantages over previously discussed technologies. One advantage derives from the flux-independent nature of ALD contributing to some relaxed reactor design-rules and scaling. Device technology is progressing at a rapid rate driving improvements of commercial deposition-equipment technology. While industry road maps for advanced and future device requirements are fairly well established, some critical applications cannot be realized by existing process technologies. For example, it is desired that commercial viability be attained for high quality dielectric laminate processes used in devices such as dielectric memory capacitors, RF products, “systems on a chip” applications, and advanced gate dielectrics with metal oxide gates.
ALD processes have often relied on solid source materials that are heated (e.g. a Knudsen thermal vaporizer source from a low vapor pressure Metal halide solid) to produce adequate precursor exposure. However, high temperature sources dictate that all manifolds located downstream to the hot sources are maintained at (or above) the source temperature. These temperatures and their maintenance are trivial to maintain throughout passive manifold components such as tubings, diffusers etc. However, valves that are necessary to produce time controlled pulsed introduction of precursors, which are key for ALD, are typically limited in service temperature, especially when corrosive precursors are involved. Accordingly, usage of many desired solid precursors poses insurmountable performance and reliability limitations on ALD manifolds deeming them inadequate for semiconductor manufacturing. Although several solid precursor delivery systems have been proposed and are implemented with more or less success in research and development, there are no known systems, thus far, that are properly suitable for high volume manufacturing. Existing systems are typically maintenance intensive, low throughput, contaminating and inefficient.
To overcome the deficiencies of conventional heated solid sources a General Metal precursor Delivery System (GMDS) technique is described in this patent. This source is relatively generic and capable of pulse delivering a variety of metal precursor into ALD reactors. Preferably, GMDS is implemented by an embodiment that maintains critical manifold components such as valves at temperatures that are compatible with low maintenance operation. Additionally, the GMDS is capable of providing high fluxes of low vapor pressure precursors. Such a system may be integrated with ALD deposition systems to enhance their capabilities.